Spin-orbit-enhanced magnetic surface second-harmonic generation in Sr2IrO4

Abstract

An anomalous optical second-harmonic generation (SHG) signal was previously reported in Sr 2 IrO 4 and attributed to a hidden odd-parity bulk magnetic state. Here we investigate the origin of this SHG signal using a combination of bulk magnetic susceptibility, magnetic-field-dependent SHG rotational anisotropy, and overlapping wide-field SHG imaging and atomic force microscopy measurements. We find that the anomalous SHG signal exhibits a twofold rotational symmetry as a function of in-plane magnetic field orientation that is associated with a crystallographic distortion. We also show a change in SHG signal across step edges that tracks the bulk antiferromagnetic stacking pattern. While we do not rule out the existence of hidden order in Sr 2 IrO 4 , our results altogether show that the anomalous SHG signal in parent Sr 2 IrO 4 originates instead from a surface-magnetization-induced electric-dipole process that is enhanced by strong spin-orbit coupling.

Three-dimensional Stacking of Canted Antiferromagnetism and Pseudospin Current in Undoped Sr$_2$IrO$_4$: Symmetry Analysis and Microscopic Model Realization

“…However, the oxygen 2p states in Sr 2 IrO 4 are much further away from (∼ 3 eV below) the Fermi level than those in the cuprates [1,34], making it disadvantageous to develop the loop-currents that requires low-energy oxygen 2p states. Furthermore, the experimental measurements [27][28][29][30] suggest a magnetoelectric loop-current order that is ferrocially stacked along the c axis, which is incompatible with the recent observation [35] of an SHG signal that switches sign every two layers.…”

“…These experimental observations support the idea that the pseudogap is associated with a symmetry-breaking hidden order. Recently, comprehensive experiments [35] conducted on undoped Sr 2 IrO 4 have ruled out the possibility of laserinduced rearrangement of the magnetic stacking, and suggest that the surface-magnetization induced electricdipole process in the SHG experiments can be strongly enhanced by SOC. However, the existence of a hidden order in Sr 2 IrO 4 remains as a possible explanation for the experimental observations of symmetry breaking and unconventional quasiparticle excitations.…”

“…It has been argued [41] that both the + + ++ and + − +− CAF can potentially explain the SHG experiment [27] without invoking the loop-currents, and either of them might have been created by the laser pump used in the experiments. The possibility of laser-induced rearrangement of the magnetic stacking, however, has been ruled out by recent comprehensive measurements [35], which show the magnetic stacking pattern is always + − −+ under the experimental condition before strong external field drives it to be + + ++.…”

Sr$_2$IrO$_4$/Sr$_3$Ir$_2$O$_7$ superlattice for a model 2D quantum Heisenberg antiferromagnet

“…Next, we further refine the structure using RA-THG, a nonlinear optical process highly sensitive to the bulk crystal symmetry through nonlinear susceptibility tensors [36][37][38]. This technique has been used for Sr 2 IrO 4 and Sr 3 Ir 2 O 7 to detect subtle structure distortions [21,27,28]. Figure 3 shows the azimuth-angle dependence of the third harmonic signals as the scattering plane is ro- tated about the c-axis, for the four different polarization configurations of the incident and reflected light, which can be either parallel (P ) or perpendicular (S) to the scattering plane.…”

“…Indeed, the single layer Sr 2 IrO 4 has reproduced much of the cuprate phenomenology: a pseudogapped metal [11][12][13], and a nodal metal with a dwave symmetric gap indicative of possible unconventional superconductivity [14,15] emerge upon electron doping from the parent phase that is approximately a Heisenberg AF [16,17]. Further, experiments indicate existence of various symmetry-breaking orders: polarized neutron diffraction [18] detects time-reversal symmetry breaking above the Néel temperature (T N ), magnetic torque [19] and second harmonic generation [20,21] measurements indicate loss of C 4 rotation symmetry, and resonant xray diffraction [22] observes splitting of a magnetic Bragg peak suggesting formation of a nearly commensurate density wave.…”